Light-emitting devices that introduce high-power solid-state laser light or high-power semiconductor laser light into a non-linear optical crystal (e.g., crystalline substances, such as LiNbO3 and KNbO3, compound semiconductor material, such as GaAs, or non-linear organic substances, etc.) to generate a second harmonic emission are well known (e.g., refer to A. Yariv: Introduction to Optical Electronics, 4th ed.; Saunders College Publishing, (Holt, Rinehart and Winston, 1991)). Further, it is also well known that when laser emissions from diode-pumped high power solid-state lasers are introduction of into birefringent uniaxial crystalline materials; e.g., such as (LiNbO3) “Lithium-Niobium-Trioxide”, (KNbO3) “Potassium-Niobium-Trioxide”, (GaAs) “Gallium-Arsenide”, and/or (ZnSSe) “Zinc-Sulfur-Selenium” the result is the emission of a frequency-doubled wavelength-halved laser emission.
In addition, it is also well known that by using the simultaneous application of the two second-order nonlinear optical processes generally termed (SFG) “Sum Frequency Generation” and (SHG) “Second Harmonic Generation” within uniaxial birefringent crystalline materials, because the birefringent crystalline materials offer large nonlinear coefficients, the results will be a frequency-doubled laser emission at normal room temperatures.
Furthermore, to construct this kind of solid-state laser device, the before mentioned birefringent uniaxial crystal has to be arranged between a pair of optical reflectors, while a single or multiple source of red-light laser emission is projected through the previously mentioned uniaxial birefringent crystal, generating therein a nonlinear second-order harmonic (i.e., a process whereby two incident photons are converted into a single photon at the second harmonic) exhibited as a frequency-doubled laser emission. These high-frequency laser emissions are then extracted from a single reflector that is transparent to the frequency-doubled emission. It is, however, the large size of these solid-state lasers, being composed of multiple components, that makes their production very costly, and their emission output very difficult to control.
Furthermore, this type of nonlinear second-order solid-state laser device takes advantage of an optical phenomenon, known generally as birefringence, to accomplish what is normally called velocity matching (i.e., sometimes called by those well versed in the art as phase-matching). Moreover, the double-refracting effects (i.e., birefringence) exhibited by most crystals can be used to phase-match both the extraordinary polarized (i.e., sometimes called E-wave polarized light) and the ordinary polarized (i.e., sometimes called O-wave polarized light) light-rays as they, upon incidence, travel through the crystalline striations that typically exist within uniaxial crystals.
For example, using an uniaxial birefringent crystal it is possible to make the ray velocity of the fundamental (corresponding to an ordinary ray or O-ray) equal (equal meaning to become phase matched) the ray velocity of the second harmonic (corresponding to an extraordinary ray or E-ray), by making sure that the birefringent uniaxial crystal was been grown from a non-congruent melt that exhibits an internal growth direction that corresponds to its optical propagation direction (the direction of phase matched ray velocities) for both the fundamental and second-harmonic emission that will propagate along a common optical axis through its structure.
Consequently, this construct will produce a phase-match between the fundamental and the second-harmonic emission; removing what normally causes the formation of interference patterns that are destructive to the generation of nonlinear second-order harmonics. However, it is very difficult and costly to execute phase-matching within the birefringent crystalline materials; e.g., such as (LiNbO3) “Lithium-Niobium-Trioxide”, (KNbO3) “Potassium-Niobium-Trioxide”, (GaAs) “Gallium-Arsenide”, and/or (ZnSSe) “Zinc-Sulfur-Selenium”, while working with the micron-sized architectures of VCSEL and related laser-diode technologies. Further, as illustrated in FIGS. 16, and 17, this type of device comprises a laser source 55, 57, 71 and non-linear optical crystals 47, 65 as being arranged between a pair of optical reflectors 51, 52, 69, 70 and where laser light is launched through the non-linear optical crystals 55, 72 to generate a second harmonic of the fundamental pump and extracted using the reflector 52, 70, which has the higher transmission for the second harmonic emission. However, the larger the device the greater is its cost of manufacturing, and since this type of laser comprises a multitude of components, it can become extremely difficult control; resulting in unstable second-harmonic emission.
In addition, other devices are well known, which extract a second harmonic from the end surface of what is normally striped GaAs or AlGaAs semiconductor lasers (see, e.g., N. Ogasawara, R. Ito, H. Rokukawa and W. Katsurashima: Jpn. J. Appl. Phys., Vol. 26 (1987) 1386), but the power of the fundamental wave occurring inside these devices is low due to a low end facet reflectivity. Additionally, the absorption loss is also large due to a stripped-laser's long cavity. These devices have even greater difficulty in achieving a phase-matching structure. Consequently, these disadvantages make it impossible to generate the second-harmonic with high efficiency. Therefore, further thought was directed towards the extraction of a second harmonic in the direction perpendicular to the cavity. See, e.g., D. Vakhshoori, R. J. Fisher, M. Hong, D. L. Sivco, G. J. Zydzik, G. N. S. Chu and A. Y. Cho: Appl. Phys. Lett., Vol. 59 (1991) 896. However, a device of the type disclosed in the above publication, the output power of the second harmonic was small, due to the fundamental emission being distributed over to wide a range, the condensing of the fundamental light was to difficult to achieve and therefore, at present the practical application for this device has yet to be realized.
Moreover, Semiconductor laser-diodes, specifically semiconductor laser-diodes with a multilayered vertical cavity (vertical orientation that is perpendicular to the substrate of the semiconductor diode) have become widely known as (VCSELs) “Vertical Cavity Surface Emitting Lasers”. Moreover, VCSEL light sources have been adopted for gigabit-Ethernet applications in a remarkably short amount of time. Further, due to their reduced threshold current, circular output beam, inexpensive and high-volume manufacture VCSELs are particularly suitable for multimode optical-fiber local-area networks (i.e., LANs).
Moreover, prior art teaches the a typical VCSEL is selectively oxidized, and therefore will contain an oxide aperture within its vertical cavity that produces strong electrical and optical confinement, enabling high electrical-to-optical conversion efficiency and minimal modal discrimination; allowing emission into multiple transverse optical-modes. Moreover, such multi-mode VCSEL lasers make ideal local area network laser light sources. VCSELs that emit in a single optical transverse mode are ever increasingly being sought-out for emerging applications including data-communication using single-mode optical fiber, barcode scanning, laser printing, optical read/write data-heads, and modulation spectroscopy.
Consequently, achieving single-mode operation in selectively oxidized VCSELs is a challenging task, because the inherent index confinement within these high-performance lasers is very large. VCSELs have optical-cavity lengths approximately one-wavelength and therefore, operate within a single longitudinal optical-mode. However, because of their relatively large cavity diameters (i.e., 5.0- to 20.0-μm diameters), these lasers usually operate in multiple transverse modes. Wherein, each transverse mode will possess a unique wavelength and what is sometimes called a transverse spatial intensity-profile (i.e., intensity pattern). Moreover, for applications requiring a small spot size or high spectral purity, lasing in a single transverse mode, usually the lowest-order fundamental mode (TEM00) is most desired.
In general, pure fundamental mode emission within a selectively oxidized VCSEL can be attained by increasing the optical loss to higher-order transverse modes relative to that of the fundamental mode. By selectively creating optical loss for any particular mode, we increase modal discrimination, which consequently leads to a frequency-doubled FCSELs operation in a single transverse optical-mode as well. Moreover, strategies for producing VCSELs that operate in single transverse optical-mode have recently been developed. Further, the previously mentioned strategies are based either on introducing loss that is relatively greater for high-order optical-modes and, thereby relatively increasing gain for the fundamental transverse optical-mode, or on creating greater gain for the fundamental transverse optical-mode; whereby increased modal loss for higher-order optical-modes has been demonstrated by three different techniques.
Moreover, the first approach to modal discrimination uses an etched-surface relief on the periphery of the top facet that selectively reduces the reflectivity of the top mirror for the higher-order transverse optical-modes. The advantage of this technique is that the etched ring around the edge of the cavity in the top quarterwave mirror-stack assembly can be produced during the VCSEL's fabrication by conventional dry-etching, or it can be post processed on a completed VCSEL die using focused ion-beam etching. A disadvantage, however, of etched-surface relief is that it requires careful alignment to the oxide aperture and can increase the optical scattering loss of the fundamental transverse optical-mode, as manifested by the relatively low (i.e., less than 2.0-mW) single-mode output powers that have been reported. Therefore, it would be more desirable to introduce mode-selective loss into the VCSEL epitaxial structure to avoid extra fabrication steps and to provide self-alignment.
Moreover, two such techniques are the use of tapered oxide apertures and extended optical cavities within VCSELs. The first approach pursued at Sadia National Laboratories (i.e., Albuquerque, N.Mex.) is predicated on designing the profile of the oxide aperture tip to preferentially increase loss to higher-order transverse optical-modes. The aperture-tip profile is produced by tailoring the composition of the (AlGaAs) “Aluminum-Gallium-Arsenide” layers, which are oxidized to create the aperture within the before mentioned VCSEL. A VCSEL containing a tapered oxide whose tip is vertically positioned at a null in the longitudinal optical standing wave can produce greater than 3.0-mW of single-mode output, and greater than 30-dB of side-mode suppression. Creating this structure, however, requires a detailed understanding of the oxidation process, and produces additional loss for the previously mentioned fundamental transverse optical-mode as well.
In addition, a second way to increase modal discrimination is to extend the optical cavity length of VCSELs and, thus increase the diffraction loss for the higher-order transverse optical-modes. Researchers at the University of Ulm (i.e., Ulm, Germany) have reported single-mode operation up to 5-mW using a VCSEL with a 4.0-μm thick cavity spacer inserted within the optical-cavity. The problem, however, is that by using even-longer cavity spacers can introduce multiple longitudinal optical-modes (i.e., causing what is sometimes called spatial hole burning), but single-transverse-optical-mode operation up to nearly 7-mW has been demonstrated. It is interesting to note that VCSELs containing multiple wavelength cavities do not appear to suffer any electrical penalty, although careful design is required to balance the trade-offs between the modal selectivity of the transverse and longitudinal optical-modes.
Finally, manipulating the modal gain rather than loss also can also produce single-mode VCSELs. A technique to spatially aperture laser gain independently of the oxide aperture has been developed at Sadia National Laboratories. The essential aspect of these VCSELs is the lithographically defined gain region, which is produced by intermixing of quantum-well active regions at the lateral periphery of the laser cavity. Typically, fabrication processes for the previously mentioned VCSELs begins with the growth of the VCSEL's bottom DBR Bragg quarterwave mirror-stack assembly onto an optical or semiconductor substrate material, the VCSEL's active-region containing the multiple quantum-well, and the VCSEL's top DBR based quarterwave mirror-stack assembly.
Moreover, the VCSEL's quantum-wells are homogenized by ion-implantation around masked regions that form laser cavities and, moreover using an epitaxial process of material deposition, like MBE or MOCVD, a second DBR quarterwave mirror-stack assembly epitaxially deposited. The resultant VCSEL has a central quantum-well active-region that preferentially provides gain for the fundamental mode. Consequently, for this approach a single-mode output of more than 2-mW with a side-mode-suppression ratio greater than 40-dB is obtained. Although, this approach requires greater fabrication complexity, it is anticipated that higher performance can be reached with further refinement of process parameters. Because of the new and greater demands of VCSEL applications, new types of single-mode VCSELs are currently under development at numerous laboratories around the world. Further, the techniques demonstrated to date introduce modal discrimination by increasing the optical loss for the higher-order modes or by increasing the relative gain of the fundamental optical-mode.
Moreover, lasers and (LEDs) “Light Emitting Diodes” are currently being used as sources of blue-light in various fields of optoelectronics, such as optical measurement, optical transmission, and optical displays. Light-emitting devices, which use LEDs (i.e., particularly those that Light-emitting devices, which use LEDs (i.e., particularly those that emit blue-light) utilizing like (GaN) “Gallium-Nitride” based semiconductors are well known (see, e.g., refer to S. Nakamura, T. Mukai and M. Senoh: Jpn. J. Appl. Phys., Vol. 30 (1991) L1998). However, since the line width of LED light is wide (i.e., a single wavelength cannot be created) lasers have in recent years been more widely used than LEDs in the field of optoelectronics. For example, with some (ZnCdSSe) “Zinc-Cadmium-Sulfur-Selenium” based semiconductor lasers, an acceptable blue-light output is obtainable (see, e.g., M. A. Hasse, J. Qiu, J. M. DePuydt and H. Cheng: Appl. Phys. Lett, Vol. 59 (1991) 1272). Nevertheless, under the present circumstances such devices can only be used upon cooling to extremely low temperatures and, therefore are impractical as light sources at a room temperature.
Moreover, semiconductor lasers and LEDs have been widely used as sources of blue light in various fields of optoelectronics; moreover, fields such as optical data-storage, optical data-transmission, optical displays, and optical measuring technologies. For example, LEDs constructed from binary semiconductor materials like GaN have been used to produce non-coherent blue-light emissions for various optoelectronic applications. However, since the before mentioned LEDs produce non-coherent light emissions, which often display wide line-width properties, the more complex semiconductor lasers, over recent years, have become more widely used than the previously mentioned LEDs in the before mentioned field of optoelectronics. For example, quaternary ZnCdSSe based semiconductor lasers can produce an acceptable output of coherent blue-light laser emissions, but only when operated at extremely low-temperatures. The problem is that they will not presently operate at the room temperatures commonly used in every day life of human beings; therefore, they have no practical application in the real world (i.e., real world meaning a world outside an environmentally controlled laboratory environment).
Typically, for a VCSEL the main photon producing structure is a double-heterostructure semiconductor Light Emitting Diode, which is often constructed from latticed-matched extrinsic semiconductor binary materials; e.g., such as (GaAs) “Gallium-Arsenide” and (GaSb) “Gallium-Antimonide”, or from latticed-matched extrinsic semiconductor ternary materials; e.g., such as (AlGaAs) “Aluminum-Gallium-Arsenide” and (InGaAs) “Indium-Gallium-Arsenide”, or from latticed-matched extrinsic semiconductor quaternary materials; e.g., such as (InGaAsP) “Indium-Gallium-Arsenic-Phosphide” and (InGaAsSb) “Indium-Gallium-Arsenide-Antimonide”.
Furthermore, prior art also teaches that a VCSEL's double-heterostructure active-region will typically contain either a (SQW) “Single Quantum Well” active-area (i.e., sometimes called a SQW laser), or a (MQW) “Multiple Quantum Well” active-area (i.e., sometimes called a MQW laser). In addition, a prior-art VCSEL will often have contra-reflecting mirrors deposited onto each of the two sides of its light emitting active-region. Typically, a prior-art VCSEL's first mirror-stack assembly, while being totally reflecting, circular in shape, and planar-flat in its deposition is deposited onto a prior-art VCSEL's reflective-base substrate layer (e.g., the substrate being typically constructed from highly reflective corundum or a nickel-aluminum alloy). While, a prior-art VCSEL's second mirror-stack assembly, being only partially reflecting, circular in shape, and planar-flat in its deposition is deposited as a prior-art VCSEL's top-most and final light reflecting structure.
Moreover, prior-art also teaches that a VCSEL will have one contra-reflecting dielectric or conductive mirror-stack assembly deposited onto each of its active-region's two opposing sides, or more succinctly, the previously mentioned prior-art VCSEL's first mirror-stack assembly would be deposited at and onto its before mentioned reflective-base substrate layer, while its second mirror-stack assembly would be deposited at the top of a prior art VCSEL's vertical cavity creating therein an optically resonate structure that amplifies into laser emissions photons, which were produced by the before mentioned prior-art VCSEL's double-heterostructure active-region's active-area.
In addition, to better understand the structural differences that lie between the present frequency doubling frequency-doubled FCSEL invention and prior art VCSEL technology, an example of a prior art VCSEL design is described below. Moreover, prior art as illustrated in FIGS. 1, 2, and 3 shows a typical example of a high frequency VCSEL design that uses the well known process of recombining ‘electron/hole’ radiation (i.e., what is sometimes called ‘radiative recombination’) to produce fundamental intra-cavity light, which is amplified into laser emissions within the VCSEL's optical-cavity. Prior art, as illustrated in FIGS. 1, 2, and 3, shows an example of a high-frequency VCSEL, which uses a metallic supporting substrate 22 (FIG. 1) as both a base-reflecting mirror structure 22 (FIG. 2), and a substrate for the subsequent growth of its multilayered structures. This is where VCSELs typically begin the epitaxial growth of multilayered materials using a well-known form of deposition such as (MBE) “Molecular Beam Epitaxy” or (MOCVD) “Metal Organic Chemical Vapor Deposition”.
Furthermore, as illustrated in FIGS. 1, 2, and 3, prior art teaches as an alternative embodiment that VCSELs using a metallic substrate 22 (FIG. 3), when made conductive, also serve as the VCSEL(s) negative electrode. The metallic substrate 22 if it comprises a (Ni—Al) “Nickel-Aluminum” alloy-mixture exhibits between 8% to 12% material lattice mismatch, or more specifically a 10% material lattice mismatch to binary GaN, which is the principal semiconductor material deposited later in many subsequent layers. Nevertheless, despite the lattice mismatch exhibited by Ni—Al it is still the preferred metallic alloy-mixture for this kind of electron conducting metallic substrate 22. Moreover, the Ni—Al metallic substrate 22 (FIG. 3) also needs to exhibit a highly reflective property as well, and should have a surface roughness less than 15 Å.
In addition, as illustrated in FIGS. 1, 2, and 3, prior art also teaches that several layers of (AlN) “Aluminum-Nitride” material can be successfully grown layer-upon-layer using MBE or MOCVD to altogether create a buffer-layer 23 (FIG. 3) having a thickness of only a few atoms, which is used for facilitating the epitaxial growth of subsequent semiconductor layers that will entirely comprise the VCSEL's structure. Further, as illustrated in FIGS. 1, 2, and 3, prior art also teaches that a VCSEL device such as the one described above has its first Bragg quarterwave mirror-stack assembly 24 grown epitaxially onto the top outermost surface of AlN comprised buffer-layers 23A, 23B, 23C, 23D (FIG. 3) using any well known epitaxial crystal growing method such as MBE or MOCVD.
In addition, as illustrated in FIGS. 1, 2, and 3, prior art also teaches a VCSEL's first quarterwave mirror-stack assembly 24 as being made from a plurality of alternating layers comprised as mirror pairs; or more precisely, a multitude of single pairs of alternating layers 24A, 24B (FIG. 3), which are constructed from (GaN/AlGaN) semiconductor materials to complete a single mirror pair. Further, a plurality of alternating layers, which will include one or more layers of N doped (GaN) “Gallium-Nitride” 24A, 24C, 24E, 24G, 24I (FIG. 3) a high-refractive semiconductor material and N doped (AlGaN) “Aluminum Gallium Nitride” 24B, 24D, 24F, 24H, 24J (FIG. 3) a low-refractive semiconductor material.
For example, as illustrated in FIGS. 1, 2, and 3, prior art teaches that a layer 24A of N doped GaN is epitaxially deposited onto the top and outermost surface of a VCSEL's last buffer-layer 23 (FIG. 3), while a layer 24B (FIG. 3) of N doped AlGaN is subsequently and epitaxially deposited onto the top and outermost surface of the VCSEL's first N doped GaN layer 24A (FIG. 3). Further, if additional mirror-pairs are required, several more layers that make-up additional mirror-pairs can be deposited onto the existing and previously deposited layers of GaN and AlGaN materials 24A, 24B, 24C, 24D, 24E, 24F, 24H, 24I (FIG. 3). Additionally, prior art also teaches that in order to increase the reflectivity of a VCSEL's first quarterwave mirror-stack assembly 24 (FIG. 3) to the required amount of total-reflectance, many additional mirror-pairs will be required, and depending on the frequency of light being reflected, as many as several hundred mirror-pairs might be needed.
In addition, prior art teaches that it should be understood that the thickness and doping levels of each deposited layer used within any VCSEL is precisely controlled. Wherein, any deviation from designed parameters, no matter how slight, would affect the performance of any VCSEL (i.e., frequency range, flux intensity). For example, as illustrated in FIGS. 1, 2, and 3, prior art teaches that a VCSEL's emitter-layer 33, if designed to emit laser-light having a wavelength of 200-nm, should have the same material thickness as each of the other remaining alternated layers that comprise both of the VCSEL's quarterwave mirror-stack assemblies 24, 32 (FIG. 3), and that this thickness dimension is to be one-quarter of one wavelength of the VCSEL's laser emission. Therefore, as illustrated in FIGS. 1, 2, and 3, a device like the one described above, having an emission wavelength of 200-nm, the thickness dimension for all layers used to comprise a VCSEL's first and second quarterwave mirror-stack assemblies 24, 32 (FIG. 3) must equal 50-nm; moreover, this dimension being equal to one-quarter of the VCSEL's emission wavelength.
In addition, prior art also teaches that the doping of the VCSEL is accomplished by the addition of various dopant materials (e.g., n-type electron donating dopants like Phosphorus and p-type electron accepting dopants like Boron) to construction material during a MBE or a MOCVD process of epitaxial deposition. Typically, a VCSEL will use many different dopant concentrations of specific dopant materials within the several different extrinsic semiconductor layers that make-up its various planar structures.
For example, the alternating layers of GaN 23A (FIG. 3) and N doped AlGaN 23B (FIG. 3), which are used to facilitate the construction of a high-frequency VCSEL's first quarterwave mirror-stack assembly 24 (FIG. 3), can be made n-type and therefore, conductive when doped with either Selenium or Silicon, using a dopant concentration ranging from 1E15- to 1E20-cm.−3, while a preferred range of doping would be from 1E17- to 1E19-cm.−3, and a nominal concentration range of doping would be from 5E17- to 5E18-cm.−3. The percentage of dopant composition exhibited by a VCSEL's first quarterwave mirror-stack assembly 24 (FIG. 3) could be stated as (Al1-xGaxN/GaN), where x represents a variable of 0.05 to 0.96, while in a preferred embodiment x would represent an amount greater than 0.8.
Therefore, as illustrated in FIGS. 1, 2, and 3, prior art teaches that once the plurality of alternating layers used in a VCSEL's first quarterwave mirror-stack assembly 24 have been deposited onto the top and outermost surface of the VCSEL's AlN buffer-layers 23, then the VCSEL's first contact-layer 25 (FIG. 3), comprising a highly n+ doped (GaN) “Gallium-Nitride” binary semiconductor material, can be epitaxially grown onto the top and outermost surface of the last alternating layer of the VCSEL's first quarterwave mirror-stack assembly 24 (FIG. 3). Further, prior art also teaches that a VCSEL's first contact-layer 25, while providing for connectivity to the VCSEL's n-metal contact 27 (FIG. 3) and to the VCSEL's n-metal contact-ring 26 (FIG. 1), also enhances the reliability of the VCSEL's design by preventing the migration of carrier-dislocations and the like to the VCSEL's active-region 28 (FIG. 3).
Furthermore, as illustrated in FIGS. 1, 2, and 3, in order to prevent overcrowding that can occur within the drawing figures, each cladding-layer is illustrated as being a single layer 28A, 28C (FIG. 3). It should also be understood that each cladding-layer could comprise a multitude of layers. Wherein, each cladding-layer 28A, 28C (FIG. 3) is epitaxially deposited upon the top outmost surface of a previously deposited layer, with each cladding-layer 28A, 28C being comprised from an N doped or P doped AlGaN ternary semiconductor material.
In addition, as illustrated in FIGS. 1, 2, and 3, prior art teaches that a VCSEL's active-region 28 (FIG. 3), which is illustrated here as being represented by a single layer, should be comprised with ether a (SQW) “Single Quantum Well”, a (MQW) “Multiple Quantum Well”, and/or some other epitaxially deposited gain-medium, all of which are well known by those well versed in the art. However, the active-region structure, regardless its form is to be epitaxially deposited upon the top and outermost surface of the VCSEL's first cladding-layer 28A (FIG. 3). The VCSEL's first cladding-layer 28A is epitaxially constructed from N doped AlGaN extrinsic ternary semiconductor material, using MBE or MOCVD to deposit the N doped AlGaN material upon the top outmost surface of the VCSEL's first contact-layer 25.
However, prior art also teaches that a VCSEL's active-region 28 (FIG. 3) could also be comprised as having one or more quantum-well cladding-layers and one or more quantum-well layers, as is typical for a MQW structure. In particular, having a first quantum-well cladding-layer and a second quantum-well cladding-layer with a quantum-well layer positioned therein, between the first quantum-well cladding-layer and the second quantum-well cladding-layer.
Furthermore, as illustrated in FIGS. 1, 2, and 3, prior art shows that a VCSEL's active-area 28B (FIG. 3) can also be comprised as having a SQW, which is constructed from (InGaN) “Indium-Gallium-Nitride” extrinsic ternary semiconductor material, using MBE or MOCVD to epitaxially deposit the InGaN material upon the top outermost surface of the VCSEL's first cladding layer 28A. While, as illustrated in FIGS. 1, 2, and 3, a VCSEL's second cladding-layer 28C (FIG. 3) is constructed from P doped (AlGaN) “Aluminum-Gallium-Nitride” extrinsic ternary semiconductor material, using MBE or MOCVD to epitaxially deposit the AlGaN material upon the top outermost surface of the VCSEL's active-area 28B.
In addition, as illustrated in FIGS. 1, 2, and 3, prior art also teaches that a VCSEL's second contact-layer 29 (FIG. 3), which is comprised from a highly p+ doped GaN extrinsic binary material, is epitaxially grown upon the top outermost surface of the VCSEL's second cladding-layer 28C (FIG. 3). Further, the VCSEL's second contact-layer 29, while providing connectivity to the VCSEL's p-metal contact 31 (FIG. 3), and to the VCSEL's p-metal contact-ring 30 (FIG. 1), will also enhance the reliability of the VCSEL's design by preventing the migration of carrier-dislocations and the like to the VCSEL's active-region 28 (FIG. 3).
Furthermore, as illustrated in FIGS. 1, 2, and 3, prior art also shows that a VCSEL's second quarterwave mirror-stack assembly 32 is also made from a plurality of alternating layers, or more precisely a multitude of mirror-pairs comprised from alternatively deposited layers 32A, 32B (FIG. 3), which are constructed from (Al2O3/ZnO) dielectric material, and used to comprise a single mirror-pair. The plurality of alternating layers, which include one or more layers of undoped (Al2O3) “Aluminum-Oxide”, being a high-refractive dielectric material (i.e., sometimes called Corundum or manufactured Sapphire) 32A, 32C, 32E, 32G, 32I (FIG. 3), and one or more layers of undoped (ZnO) “Zinc-Oxide”, being a low-refractive dielectric material 32B, 32D, 32F, 32H (FIG. 3).
For example, as illustrated in FIG. 1, FIG. 2, and FIG. 3, prior art teaches that a layer of Al2O3 32A is deposited upon the top outmost surface of a VCSEL's second contact-layer 29 (FIG. 3), while a layer of ZnO 32B (FIG. 3) is subsequently deposited upon the top outmost surface of the VCSEL's first Al2O3 layer 32A (FIG. 3). Further, if additional mirror-pairs are required several more layers can be used to make-up additional mirror-pairs by depositing them onto the existing layers of Al2O3 and ZnO material 32A, 32B, 32C, 32D, 32E, 32F, 32G, 32H, 32I (FIG. 3). Moreover, in order to increase the reflectivity of a VCSEL's second quarterwave mirror-stack assembly 32 (FIG. 3) to the required amount of partial-reflectance, many additional mirror-pairs may be required, and depending upon the frequency of light being reflected, as many as several hundred might be used to create a single mirror-stack assembly. Additionally, as illustrated in FIGS. 1, 2, and 3, prior art also teaches that a VCSEL's last layer to be deposited is an emitter layer 33, which is constructed from a high-refractive dielectric material such as undoped ZnO, using any well known method deposition, such as MBE or MOCVD to deposit the material.
In addition, as illustrated in FIGS. 1, 2, and 3, prior art also shows that a VCSEL's p-metal contact 31 (FIG. 3), and the VCSEL's p-metal contact-ring 30 (FIG. 3) are both formed upon the top outermost surface of the VCSEL's second contact-layer 29 (FIG. 3), by disposing any suitable conductive material, such as Indium-Tin-Oxide, Gold, Zinc, Platinum, Tungsten, or Germanium metallic alloys. Further, as illustrated in FIGS. 1, 2, and 3, prior art continues to show that a VCSEL's n-metal contact 27 (FIG. 1), and the VCSEL's n-metal contact-ring 26 (FIG. 2) are both formed upon the top outermost surface of the VCSEL's first contact-layer 25 (FIG. 3), by disposing any suitable conductive material, such as Indium-Tin-Oxide, Gold, Zinc, Platinum, Tungsten, or Germanium metallic alloys thereon. Further, it should be understood that a chosen method of material deposition depends upon the material being selected for a VCSEL's electrical contacts 26, 27, 30, 31 (FIG. 3) and therefore, specific methods of disposition, disposing, and patterning onto the VCSEL's first and second contact-layers 25, 29 (FIG. 3) for any specific material, must be considered in the construction of the VCSEL's electrical contacts 26, 27, 30, 31 (FIG. 3).
Furthermore, as illustrated in FIGS. 1, 2, and 3, prior art also teaches that a VCSEL's second contact-layer 29 (FIG. 3), a VCSEL's second cladding-region 28C, a VCSEL's active-area 28B, and a VCSEL's first cladding-layer 28A (FIG. 3) are shown as being mesa-etched, which defines the overall shape and structure of the VCSEL, while displaying the VCSEL's diameter dimensions to be substantially larger than the VCSEL's topmost deposited emitter layer 33 (FIG. 1) and second Bragg quarterwave mirror-stack assembly 32 (FIG. 2). Further, after completing a mesa-etching process a VCSEL's p-metal contact 31 (FIG. 3), and the VCSEL's p-metal contact-ring 30 are deposited upon the top outermost surface of the VCSEL's second contact-layer 29, leaving the VCSEL's emitter-layer area open 33 (FIG. 3). Additionally, prior art also teaches an alternative embodiment, where the deposition of a VCSEL's n-metal contact 27 is made upon the top outermost surface of the VCSEL's NiAl based metallic substrate 22 (FIG. 3). Moreover, deposition of a VCSEL's n-metal contact 27 will provide for a metallic substrate 22 that functions additionally as an electrically negative contact-layer.
Furthermore, as illustrated in FIGS. 1, 2, and 3, prior art also teaches that a VCSEL's metallic substrate 22 (FIG. 3), when used in conjunction with a highly reflective AlGaN/GaN quarterwave mirror-stack assembly 24 provides approximately 99.99% of the VCSEL's total reflectivity. Further, prior art teaches further that a group of VCSELs can also be configured and manufactured as an array of laser-diodes. For example, as illustrated in FIGS. 4, 5, and 6, a group of VCSELs are shown as being grouped together into a single file linear configuration.
In addition, prior art also teaches a frequency-doubled second-harmonic generating VCSEL laser-diode, which is constructed from compound semiconductor material that comprise elements listed in columns III-V of the ‘Periodic Table of Elements’. For example, using a method of epitaxial deposition, a compound semiconductor material such as GaAs, AlGaAs, GaInAsP, AlGaInP, GaInAsP, etc. may be utilized as construction material during the deposition of a VCSEL's active-region. Further, a compound semiconductor material, such as GaAs, GaAsP, etc. may be utilized as material that provides for a lattice matched substrate wafer. Further, prior art also teaches that a frequency-doubled second-harmonic generating VCSEL laser-diode will be comprised as a multitude of layered structures; e.g., a cavity, the reflectors, etc. Wherein, the multitude of layered structures are epitaxially deposited layer-upon-layer onto the top outermost surface of a GaAs comprised substrate wafer exhibiting the necessary crystalline orientation of [111].
Moreover, a first electrode is typically formed upon the bottom surface of the wafer substrate, while a first reflector, comprising a multitude of semiconductor layers is epitaxially grown upon the upper surface of the previously formed first electrode. Typically, any semiconductor crystalline material used to form a VCSEL's cavity, as a result of epitaxial growth, will have the same crystalline orientation exhibited by the wafer substrate the material was grown upon. Therefore, the crystalline orientation of the latter determines that of the former.
In addition, what is formed upon the upper surface of the first reflector is typically a spacer-layer, an active-layer and a second spacer-layer. Further, an insulation layer is typically formed between the first electrode and the spacer-layer. Further, a second electrode is formed upon the upper surface of the substrate, and the substrate is finally etched to form a second harmonic output port on the side of the second electrode. Further, in the second harmonic output-port, a phase-matching layer is formed on the upper side of the second spacer-layer, and a reflector for output of the second harmonic on the upper surface of the phase-matching layer is paired with the reflector.
Moreover, a frequency-doubled second-harmonic generating VCSEL will comprise two DBR reflectors and the layers formed between them will comprise a first spacer-layer, an active-layer, a second spacer-layer, and a phase-matching layer. Typically, a frequency-doubled second-harmonic generating VCSEL laser-diode has a phase-matching layer that does not lie between the two electrodes, but in a section through which no current flows. Therefore, due to an increase in electric resistance, disadvantages such as a decrease in the power exhibited by the fundamental wave do not arise.
In addition, prior art also teaches that a frequency-doubled second-harmonic generating VCSEL's second DBR reflector is constructed by depositing in succession, two dielectric materials (e.g., alternating layers of TiO2 and SiO2) that exhibit diametrically opposed refractive indices and used to form the second DBR reflector at the output emission side of a frequency-doubled second-harmonic generating VCSEL laser-diode. The dielectric thin films are used to construct about 10 mirror-pairs (or more depending on the process used) by depositing in succession, each dielectric thin film layer upon the other; alternating the deposition from the material with a high index of refraction to the material with a low index of refraction (e.g., alternating from TiO2 to SiO2). Further, the layers (alternating TiO2 and SiO2 layers) used to form a top DBR based reflector would have a thickness t that can be expressed as—Equation 1:td1=λ1/[4nd1(λ1)], andtd2=λ1/[4nd2(λ1)], respectively.
Wherein, n(λ) is the refractive index at the wavelength λ, while λ1 is the wavelength of the fundamental wave. Further, the subscript d1 denotes the film thickness and refractive index for SiO2, while the subscript d2 denotes the film thickness and refractive index for TiO2.
In addition, prior art also teaches that a frequency-doubled second-harmonic generating VCSEL laser-diode has a phase-matching layer that is also configured as a superlattice comprising of two layers of AlGaAs, but each layer having different amounts of Al each layer represented by subscript s1 and subscript s2, respectfully (content of Al for s1 equals 50-90%, while content of Al for s2 equals 10-50%). Moreover, the thickness ts1, ts2 of each layer s1, s2 is determined using each layer's respective reflective indice ns1(λ) and ns2(λ), as demonstrated in the following equation—Equation 2:ts1=[ns1(λ2)/λ2−2s1(λ1)/λ1]−1/2ts2=[ns2(λ2)/λ2−2ns2(λ1)/λ1]−1/2
Wherein, the total thickness of the phase-matching layer is but a few times greater than the reciprocal of the average absorption constant; i.e., having a deposition thickness that lies somewhere between 2- to 20-μm. In the above example, the substrate orientation was [111] (i.e., a substrate having a One-One-One crystalline orientation). The present invention is not limited, however, to the crystalline orientation of [111].
Furthermore, by using a first non-linear process, which is sometimes called sum-frequency generation by those well versed in the art, a second-harmonic emission of two fundamental waves may be generated, albeit with a difference inefficiency, using a III-V comprised substrate wafer that exhibits a crystalline orientation of [100] (i.e., a substrate having a One-Zero-Zero crystalline orientation that differs greatly from that of the One-One-One crystalline orientation previously mentioned). Consequently, to maintain phase-matching for the present invention the crystalline orientation exhibited by its substrate wafer must not exceed an angle of more than 5° with respect to the crystalline orientation of [100].
Alternatively, prior art also teaches that the active-layer may be composed of AlGaAs, etc., its composition being determined according to the wavelength of the fundamental wave. The composition of the two DBR reflectors is not necessarily restricted to dielectric multi-layered films, and may be based upon semiconductor multi-layered films, metallic films, or a combination of metallic and dielectric films. The second DBR reflector on the output end needs only to have a reflectivity high enough to cause lasing of the fundamental wave, and a transmissivity high enough to extract the generated second harmonic. The first DBR reflector on the non-output end need only have a reflectivity high enough to cause lasing of the fundamental wave.
Moreover, a semiconductor multi-layered film base reflector may be used as the second DBR reflector because its electrical resistance is lower than that of the dielectric multi-layered film. For example, the superlattice of the phase-matching layer was composed of an AlGaAs compound semiconductor, but it may be composed of any III-V compound semiconductor, such as InGaAs, AlGaInP, GaInAsP, etc.
Alternatively, prior art also teaches that the VCSEL's second DBR reflector may also be composed of an II-VI group compound semiconductor such as ZnCdSSe, ZnSSe, ZnCdS, etc. Prior art teaches, when the phase-matching layer is formed utilizing elements from columns II-VI of the ‘Periodic Table of Elements’ the width of the phase-matching layer may be increased to a degree which is allowed by the fabrication process and which will support the lasing of the fundamental wave within a VCSEL. Prior art also teaches that besides the above embodiments, any well-known method (see, for example, those shown in the reference by A. Yariv and the reference by D. Vahkshoori referred previously) may be employed to realize phase matching. For example, the second harmonic may also be effectively generated even where the thickness of layers corresponding to Equation 2 takes the following values—Equation 3:ts1=[ns1(λ2)/λ2]1/2ts2=[ns2(λ2)/λ2]−1/2
Wherein, this method is well known to realize phase-matching in standing waves. While those described in Equation 1 use modulation in the value of the non-linear coefficients, but phase-matching is also possible through changing signs of the non-linear coefficients.